Skin cancer is the most commonly occurring of all cancers, accounting for more than a million incidences in the United States annually [1]. There are three types of skin cancer including basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), and malignant melanoma.
BCC arises from transformed epidermal stem cells in the basal layer of the epidermis, such that upon pathological analysis, tumor cells have a characteristically columnar appearance. BCC tends to form nests of the tumor cells whose peripheral borders appear as an orderly line of basal cells (termed “peripheral pallisading”) and whose centers possess similar cell types with low amounts of cytoplasm. Clinically, BCC presents in four variants: superficial, nodular, pigmented, and sclerosing [2]. Clinical recognition of the BCC variants can be difficult because of their resemblance to other clinical entries such as sebaceous hyperplasia and molluscum contagiosum (nodular BCC) or eczema and psoriasis (superficial BCC).
SCC originates from transformed cells in the stratum spinosum, and has the potential to develop downwards and invade into underlying structures. SCC appears pathologically different from BCC. The tumor nests in SCC are bordered by polygonal cells with larger degrees of cytoplasm than that in BCC. SCC tumor cells also differentiate towards the center of the nests such that the nest centers often have sections of keratinized epidermal cells which appear as “keratinized pearls” [3]. Clinical differentiation of SCC from other skin lesions also normally requires histo-pathological analysis as they can often be confused with keratoacanthomas.
Melanomas arise from transformed melanocytes in the basal layer of the epidermis. The cancerous cells then spread upwards into the epidermis as well as downwards into the dermis and underlying tissues. Clinically, melanomas appear as dark pigmented lesions with four subtypes: Lentigo maligna, superficial spreading melanoma, acral lentiginous melanoma, and nodular melanoma. The first three melanomas initially spread horizontally for years before vertical invasion, and therefore have an excellent prognosis if diagnosed early enough. Nodular melanoma, however, almost immediately begins a vertical growth phase and is therefore associated with a much worse prognosis.
Currently, the gold standard for skin cancer diagnosis is histopathological evaluation of tissue biopsies. Depending on the nature of the skin lesion, different types of biopsies are performed. For protruded skin lesions believed not to be melanocytic, a shave biopsy is performed. For flat lesions and lesions where it is imperative that a full-thickness specimen is taken, a punch biopsy may be advantageous. Almost all small lesions suspected to be melanocytic, however, are removed via excisional biopsy. The excisional biopsy removes the entire lesion including a margin of normal skin at a depth that extends into the subcutaneous tissue. This is to ensure proper analysis of the depth profile of the lesion as well as a precautionary method. In suspicious lesions that are potentially non-melanomas, incisional biopsies are taken where only a portion of the lesion is removed for histological analysis.
Although the current gold standard for evaluation of potentially cancerous skin lesions is effective, it is far from perfect. Biopsy is an invasive and painful procedure that is often unnecessary when the lesions are benign. The biopsy is also a time-consuming and expensive procedure. Additionally, in patients who present with multiple similarly appearing lesions, the selection of the site for biopsy is subjective and could result in misdiagnosis [2]. Thus, there remains a need for a non-invasive technique that can reliably analyze suspicious lesions in situ in real-time and serve as a method to initially guide biopsy and eventually potentially serve as a replacement for histo-pathological analysis.
Raman spectroscopy (RS) is an optical technique that probes the specific molecular content of a sample by collecting in-elastically scattered light. Raman spectroscopy is a regularly used tool in analytical chemistry to determine the presence of specific molecules in mixed samples. Recent studies have shown RS can be utilized for investigating cancerous human tissues. Examples include the analysis of tissues from the breast [4-7], cervix [8,9], bladder and prostate [10], lung [11], and GI tract [12]. Although Raman spectroscopy has been proven to be a powerful tool for the biochemical analysis of tissue, a significant limitation lies in its inability to practically relate biochemical data to tissue structure.
Optical coherence tomography (OCT) is a recently developed imaging modality capable of generating cross-sectional images of tissue micro-structure [13], function [14], and optical properties [15-17]. Clinical applications include imaging of the retina [18] and anterior chamber of the eye [19-22], GI tract [23], coronary vasculature [24]. OCT has also been applied to the in vivo imaging of the skin to examine dermatitis, psoriasis, as well as the effect of varies ointments, water, tape stripping, and UV radiation on normal skin [25-29]. Additionally, the polarization-sensitive OCT has been employed to assess burn depth based on changes in dermal collagen structure [30-32]. Applications of OCT for the imaging of skin cancers, however, have been limited. Initial studies have demonstrated OCT's ability to image tumor nests in basal cell carcinomas, as well as the structure of the horny cysts in seborrheic keratosis [25]. Additional studies have shown the appearance of melanocytic nevi and their correlation with histopathology [33, 34]. However, while OCT is capable of producing cross-sectional images of tissue microstructure with near histological resolution, conventional application prohibits determination of the biochemical composition of tissue anomalies and defects.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.